Cesium-137 constitutes a major source of radioactivity in nuclear waste streams, such as the high level liquid wastes of nuclear fuel reprocessing solutions. Selective removal of this radionuclide from the aqueous acidic nuclear waste streams would greatly simplify their handling and ultimate disposal.
Various processes for the separation of cesium ions are known in the art. These processes include selective ion precipitation, ion exchange using solid materials, and solvent extraction processes that are also referred to as liquid/liquid separation processes.
For separating cesium ions from other alkali metal ions in very basic solution, solvent extraction methods have been known for many years. Ross and White, Anal. Chem., 36:1998 (1964); Egan, et al., Inorg. Chem., 4:1055 (1965); Arnold, et al., Ind. Eng. Chem. Process. Des. Dev., 4:249 (1965); and Roddy and Coleman, Inorg. Nucl. Chem., 35:4271 (1973). Other solvent extraction methods for cesium ion separation have been reported in the art. Kyrs et al, Coll. Czech. Chem. Commun., 25:2642 (1960); Slater, Nucl. Sci. Eng., 17:576 (1963); Crowther and Moore, Anal. Chem., 35:2081 (1963); Rais, et al., J. Inorg. Nucl. Chem., 38:1376 (1976); and Koprda, et al., J. Radioanal. Nucl. Chem., 80:55 (1983).
More recently, solvent extraction methods have been reported for the recovery of cesium ions from acidic solutions. Blasium and Nilles, Radiochem. Acta, 36:207 (1984); Gerow, "The Use of Macrocyclic Polyethers to Remove Cesium-137 from Acidic Nuclear Wastes by Solvent Extraction", Doctoral Dissertation, University of South Carolina, Columbia, S.C. (1980); Gerow et al., Sep. Sci. Technol., 16(5):519-548 (1981).
None of the processes described to date has proven entirely satisfactory when cost, safety, engineering and performance considerations are all taken into account. Schulz and Bray, Sep. Sci. Technol., 22:191 (1987).
Several factors affect the choice of a system for a solvent extraction process to separate cesium ions from aqueous acidic media. These factors include the choice of the cesium ion extractant, the composition of the aqueous and organic phases ("the extraction system"), the chemistry of the extraction process, and the practicality of the full-scale process.
The cesium ion extractant should be soluble enough in the organic solution ("organic phase") to provide a sufficiently high extractant concentration, and thereby cesium ion concentration in the organic phase. The cesium ion extractant should be relatively insoluble in the acidic aqueous solution ("aqueous phase"). It should be possible to separate the cesium ions from the extractant after use to permit recycling of the cesium ion extractant and recovery of the cesium ions.
The extraction system should be such that the distribution of cesium ions between the organic and aqueous phases (denoted by the distribution ratio, D.sub.Cs, described in detail hereinafter in "Materials and Methods") heavily favors the organic phase over the acidic aqueous phase to permit complete removal of the cesium ions from the acidic aqueous solution. The extraction system should also be such that the distribution of cesium ions between the organic and aqueous phases favors the aqueous phase over the organic phase under extractant recycling and cesium ion recovery conditions. The extraction system should further permit sufficient extraction of cesium ions in the presence of competing ions in the aqueous phase. The extraction system should be easy to handle and control, as well as be chemically compatible with nuclear fuel reprocessing processes.
One of the major problems encountered in the development of a workable liquid/liquid extraction process for the separation of cesium ions from aqueous acidic media is the limited solubility of many potential cesium ion extractants in solvents of low polarity that constitutes the organic phase. Another major problem is the loss of the cesium ion extractant due to its solubility in the aqueous phase. Yet another major problem is finding an extractant that permits efficient and selective extraction of the cesium ions from solutions containing high concentrations of common mineral acid anions, such as nitrate and chloride. Few processes have been able to satisty this last problem. Schulz and Bray, Sep. Sci. Technol., 22:191 (1987).
The chemistry of the cesium ion further complicates the solution of these problems. The cesium(+1) ion has a low charge density due to its large ionic radius and low charge. As a result, the energy associated with bond formation between the cesium ion and the functional groups of the organic extractants is typically insufficient to completely dehydrate the cation and to strip away the water molecules associated with the anion that must accompany the cation into the organic phase to maintain electrical neutrality. Schulz and Bray, Sep. Sci. Technol., 22:191 (1987). For this reason, liquid/liquid cesium ion extraction usually involves the transfer of a complex bearing a number of associated water molecules into an organic solvent.
To compensate for this charge density effect, several workers have proposed the use of macrocyclic polyethers ("crown ethers") as extractants. Kinard, et al., Sep. Sci. Technol., 15:1013 (1980); Kinard and McDowell, J. Inorg. Nucl. Chem., 43:2947 (1981); McDowell, et al., Solvent Extr. Ion Exch., 4:217 (1986); Blasius and Nilles, Radiochim. Acta, 36:207 (1984); Blasiums and Nilles, Radiochim. Acta, 35:173 (1984); Shuler, et al., Solvent Extr. Ion Exch., 3:567 (1985); McDowell, Sep. Sci. Technol., 23:1251 (1988); and McDowell, et al., Anal. Chem., 64:3013 (1992). Because the interaction of the crown ether with the ion involves little change in the conformation of the extractant molecule, the energetics of extraction are somewhat more favorable.
Other workers concluded that it was not possible to extract cesium ions into an acceptable diluent using crown ethers alone from a medium containing high concentrations of mineral acids, despite the improved energetics. Gerow et al., Sep. Sci. Technol., 16(5):519-548 (1981); Schulz and Bray, Sep. Sci. Technol., 22:191 (1987).
Several approaches have been taken in the art to enhance the liquid/liquid separation of cesium ions from acidic media by crown ethers. These approaches usually involve providing an organophilic counterion in some form to balance the charge of the cesium ion in the organic phase, thereby avoiding the need to transfer an inorganic anion and its associated water molecules from the aqueous phase. Two approaches involving providing an organophilic counterion in some form are discussed below, as is one other approach.
One approach to enhance cesium ion extraction by crown ethers is to attach proton-ionizable groups to the crown ethers to produce a molecule which is both a coordinator and a counterion. Strzelbickl and Bartsch, Anal. Chem., 53, 1894 (1981); Brown and Bartsch, "Ion Extraction and Transport by Proton-Ionizable Crown Ethers" in Inclusion Aspects of Membrane Chemistry, T. Osa and J. L. Atwood, eds., Kluwer Academic Publishers (Boston: 1991), pp. 1-57. A disadvantage of this approach is that the proton-ionizable crown ethers described thus far, for example, are unsuitable for extractions involving strongly acidic aqueous phases because they are in protonated form under those conditions instead of anionic form. Schulz and Bray, Sep. Sci. Technol., 22:191 (1987). Another disadvantage is the difficulty in synthesizing the crown ethers with bonded proton-ionizable groups.
Another approach to enhance cesium ion extraction by crown ethers involves using high molecular weight organic acids (that have inherent extractant capability) along with a crown ether in the diluent. Kinard, et al., Sep. Sci. Technol., 15:1013 (1980); Kinard and McDowell, J. Inorg. Nucl. Chem., 43:2947 (1981); McDowell, et al., Solvent Extr. Ion Exch., 4:217 (1986); Shuler, et al., Solvent Extr. Ion Exch., 3:567 (1985); McDowell, et al., Anal. Chem., 64:3013 (1992); and McDowell, et al., Sep. Sci. Technol., 18:1483 (1983). A disadvantage of the organic acid approach is that, although combinations of organic acids and crown ethers sometimes permit satisfactory cesium ion extraction, they often fail to permit the complete recovery of the cesium ions. Schulz and Bray, Sep. Sci. Technol., 22:191 (1987). A practical drawback is the complicated chemistry involved with multiple extractants, resulting in limited extraction efficiency at high acidity and inefficient cesium ion recovery.
Regarding the selection of the crown ether extractant, the art has made several general observations: (i) that branched side chains on crown ether benzo derivatives increase the solubility of the extractant in the organic phase (Gerow et al., Sep. Sci. Technol., 16(5):519-548 (1981)); (ii) that increasing the side chain length on crown ether benzo derivatives tends to increase their solubility in the organic phase up to 7 carbons in length, but decreases the solubility in the organic phase when the side chain length is more than 7 carbons (Gerow et al., Sep. Sci. Technol., 16(5):519-548 (1981)); (iii) that the strongest ion binding by a crown ether occurs when the ion fits best into the crown ether cavity, unless a "sandwich" complex is formed (two crown ethers to one metal ion) (McDowell, et al., Anal. Chem., 64:3013-3017 (1992)); (iv) that 21-crown-7 is the appropriate size for cesium ions (McDowell, et al., Anal. Chem., 64:3013-3017 (1992)); and (v) that dibenzo crown ethers are selective for alkali metals (such as cesium), whereas dicyclohexano crown ethers are selective for alkaline earth metals (such as strontium) (McDowell, et al., Anal. Chem., 64:3013-3017 (1992)).
McDowell et al., Anal. Chem., 64:3013-3017 (1992), reported that bis-4,4'(5')-(t-butyl benzo)-21-crown-7 (Compound I8, below) yields satisfactory cesium ion extraction (D.sub.Cs .congruent.100) from acidic nitrate media (approximately 0.1M HNO.sub.3) by a synergistic effect when combined with an appropriate cation exchanger (a lipophilic counterion, such as didodecylnaphthalene sulfonic acid) in toluene. Dicyclohexano-18-crown-6 also extracted cesium ions from acidic aqueous solution in the same extraction system, but the cesium ion distribution ratio was lower (D.sub.Cs .congruent.43 at 0.1M HNO.sub.3). McDowell, et al., Anal. Chem., 64:3013-3017 (1992).
Compound I8 has the bis-4,4'(5')-(benzo)-18-crown-6 backbone structure of Formula I, with the benzo substituent side chain X structure of Formula 8, where the wavy line denotes the bond to the benzo ring. ##STR1##
Gerow et al., Sep. Sci. Technol. 16(5):519-548 (1981), reported the extraction of cesium ions from aqueous nitric acid media using the macrocylic polyether, bis-4,4'(5')-(1-hydroxyheptyl-benzo)-18-crown-6. Those authors concluded that the crown compound alone was not a sufficiently strong complexing agent to extract cesium ions from an acidic aqueous medium containing inorganic anions such as nitrate or chloride. However, when a large organic counterion, such as didodecylnaphthalene sulfonic acid or di-2-ethylhexyl phosphoric acid was included in the organic phase, cesium ions could be extracted from a 3M nitric acid solution.
Davis, Jr. et al., U.S. Pat. No. 4,749,518 report the addition of bis-4,4'(5')-(1-hydroxyheptyl)-benzo)-18-crown-6 to enhance cesium ion extraction by a cation exchanger (didodecylnaphthalene sulfonic acid or dinonylnaphthalene sulfonic acid at 5 volume percent) in an organic diluent (27-50 volume percent tri-n-butyl phosphate and 68-45 volume percent kerosene) for the separation of cesium ions from aqueous acidic solutions. Davis, Jr. et al., above, reported cesium ion distribution ratios of about 1.5 for bis-4,4'(5')-(1-hydroxyheptyl-benzo) 18-crown-6 ether (0.05M) in the presence of competing ions in an acidic nuclear waste model solution (about 3M HNO.sub.3). Those workers reported recovery of up to about 80 percent of the cesium ions from the organic phase through back-extraction into 1M HNO.sub.3 aqueous solution. Davis, Jr. et al. also showed that using increasing concentrations of the crown ether increased the distribution ratio.
A disadvantage of the two approaches involving providing an organophilic counterion in some form is that under very acidic conditions, such as nuclear waste processing solutions, the would-be counterions are predominantly in their protonated forms, unable to neutralize the charge of the cesium cation.
A third approach to enhance cesium ion phase transfer using crown ethers was suggested from work with strontium, where it was found that in the presence of substantial quantities of dissolved water in the aliphatic diluent, a lipophilic anion was not required. Horwitz, et al., Solvent Extr. Ion Exch., 8:199 (1990). That report showed that the strontium ion extraction efficiency increased with increasing amounts of water dissolved in the organic phase. Water was effectively dissolved in the organic phase by using any of a variety of oxygenated, aliphatic solutions (e.g. ketones, alcohols) as the organic phase diluent, and equilibrating the organic phase with aqueous solution before use. Horwitz, et al., Solvent Extr. Ion Exch., 8:199 (1990).
The observations with strontium were recently extended to cesium ion extraction. Dietz, et al., Solvent Extr. Ion Exch., 14:1-12 (1996), reported finding that with an appropriate diluent, cesium ions can be extracted from acidic nitrate media using crown ethers in the absence of an organic counterion (either incorporated into the crown ether or as an organic acid in solution). Dietz, et al., studied the cesium ion extraction behavior of bis-4,4'(5')-(t-butyl-benzo) and bis-(cyclohexano) derivatives of 18-crown-6, 21-crown-7, and 24-crown-8. They observed that the cesium ion extraction with Compound I8, bis-(t-butyl-benzo)-18-crown-6, was insensitive to the dissolved water concentration, because a cesium ion is extracted by the crown ether as a 2:1 sandwich complex. Dietz, et al., Solvent Extr. Ion Exch., 14:1-12 (1996).
The oxygenated diluents studied by Dietz, et al., ranged from single functional group alcohols and ketones to carboxylic acids. Dietz, et al., Solvent Extr. Ion Exch., 14:1-12 (1996). Those authors concluded that ketones were the best organic diluent because the cesium ion distribution ratios for the same crown ether were the highest in the ketones, and the crown ether concentrations were higher in the ketones.
An important practical advantage to the approach with crown ether alone over the approach with the added counterion is that the process chemistry using a single extractant is greatly simplified, so that only the nitric acid concentration need be changed to control the cesium ion extraction.
Dietz, et al., above, studied the cesium ion extraction capability of several bis-4,4'(5')-(substituted-benzo)-crown ethers with no added organic acid counterion. Among their other findings, those authors reported that although the bis-substituted-18-crown-6 ethers had higher distribution ratios for cesium ions in the extraction system, the larger crown ethers showed greater cesium selectivity over sodium, superior functional stability, and better compatibility with PUREX-like diluents.
Although the need for improved methods for the removal and recovery of radioactive cesium ions from acidic nuclear waste streams has long been apparent, most of the cesium ion extraction systems studied to date are likely to be of only limited practical value, for various reasons. For example, the physical and chemical properties of certain oxygenated, aliphatic diluents preclude their use in process-scale applications (e.g. the flash point of methyl isobutyl ketone). Further, the aliphatic crown ethers that are readily soluble in oxygenated, aliphatic diluents yield only low cesium ion distribution ratios (e.g. di-cyclohexano-18-crown-6 D.sub.Cs .congruent.0.1 in the absence of organophilic counterion). Still further, the aromatic crown ethers that provide more efficient cesium ion extraction have limited solubilities in oxygenated, aliphatic diluents.
Therefore, there remains a need for cesium ion extraction systems that combine an efficient and selective cesium ion extractant with a diluent possessing satisfactory physical and chemical properties that meet the criteria discussed above. The discussion that follows provides one solution to the cesium ion separation problem.